As shown in FIG. 8, a decanter-type centrifugal separator 1 used for solid-liquid separation has a hollow bowl 2 and a screw conveyor 3 disposed coaxially with the bowl 2 inside thereof. The separator 1 is configured such that where the bowl 2 and the screw conveyor 3 are rotated at a high speed, the raw liquid to be processed introduced into the bowl 2 through a feed tube 4 is separated by a centrifugal force into a liquid (liquid phase) and a solid content (solid phase), and the two phases are individually discharged into a liquid recovery system and a solid recovery system.
More specifically, where the raw liquid to be processed is introduced into the bowl 2 rotated at a high speed, the solid content, which has a high density, precipitates to a position close to the inner circumferential surface of the bowl 2 under the effect of the centrifugal force, the liquid, which has a low density, is positioned on radially inward of the precipitated solid content, and a state is assumed in which the solid phase on the outer side is separated from the liquid phase on the inner side. The precipitated solid content is transported by the screw conveyor 3 rotating with a predetermined difference in speed with respect to the bowl 2 toward a solid discharge port 5 formed at one end of the bowl 2 (left end in FIG. 8), and the solid content is discharged from the discharge port 5 to the outside of the bowl 2 (to the solid recovery system). Meanwhile, the separated liquid is discharged from a liquid discharge port 6 formed at the opposite end of the bowl 2 (right end in FIG. 8) to the outside of the bowl 2 (to the liquid recovery system). A dam 7 determining the level of liquid is disposed in the liquid discharge port 6, and the separated liquid is discharged by overflowing the top (on radially inward) of the overflow edge of the dam 7.
In the decanter-type centrifugal separator 1 of this type, very large drive energy (electric power) is necessary to rotate the bowl 2 and the screw conveyor 3 at a high speed. Therefore, a variety of measures have been used to reduce energy consumption.
For example, US 2004/0072667 A1 and US 2004/0072668 A1 disclose a centrifugal separator configured such that a hole passing through a bowl is formed in a dam or in the vicinity thereof, a jet nozzle is attached so as to protrude from the through hole to the outside of the bowl and so as to be open in the direction opposite to the rotation direction of the bowl, and the separated liquid in the bowl not only overflows from above the overflow edge of the dam, but is also discharged from the jet nozzle. In such a centrifugal separator, a counterforce generated when the separated liquid in the bowl is jetted out from the jet nozzle can be used as an auxiliary force for rotationally driving the bowl. As a result, energy consumption for rotationally driving the bowl can be reduced.
Further, JP 2010-525945 A discloses a centrifugal separator configured such that a casing (33) forming a discharge opening (36) which is open in the direction opposite to the rotation direction of the bowl is attached to a liquid discharge port, instead of attaching a dam to the liquid discharge port, and the separated liquid overflowing the top of an overflow edge (39) of a dam plate (45) disposed in the discharge opening (36) is discharged in the direction opposite to the rotation direction of the bowl. In such centrifugal separator, the energy of the rotating separated liquid can be obtained again in the discharge opening. As a result, when the separated liquid is discharged in the direction opposite to the rotation direction, power within a range of 10 to 15% can be obtained.